A heat pump that makes new noises is usually telling you something specific. The good news is that in San Diego, where heat pumps run more annual hours than central air conditioners because we use them for both cooling and mild winter heating, the noise vocabulary is short. Four sound categories cover roughly 90% of what we hear on service calls. One of them is normal. The other three need attention.
The fast answer
| Sound | Likely cause | Call now? |
|---|---|---|
| Whoosh during mode change (5-15 sec) | Reversing valve switching, normal | No |
| Loud bang at startup or shutdown | Failing compressor mount or slugging | Yes |
| Grinding or rattling from outdoor fan | Debris in the fan blade or a loose blade | Yes, shut off first |
| High-pitched squeal at startup | Failing motor bearings or belt issue | Within a week |
| Constant electrical hum with no airflow | Failed contactor or seized fan motor | Yes |
| Hissing near refrigerant lines | Refrigerant leak | Yes |
If the noise is loud, mechanical, and new, shut the system off at the thermostat until a tech can look at it. A failing fan motor or loose blade can destroy a coil in minutes, and that turns a $400 repair into a $2,500 one.
1. The whoosh during defrost (this one is normal)
When a heat pump in heating mode switches into a defrost cycle, the reversing valve flips the refrigerant flow direction. This produces a sudden whoosh or rush of air noise that lasts five to fifteen seconds. You may also hear the indoor blower briefly slow or pause.
This happens roughly every 30 to 90 minutes in cold weather. In San Diego it mostly shows up on inland mornings in Escondido, San Marcos, Vista, El Cajon, and Santee from December through February, when outdoor temperatures dip below 40F. Coastal homes in Carlsbad, Encinitas, La Jolla, and Coronado rarely hear it because morning lows are warmer.
What to do. Nothing. If the whoosh is brief and the system returns to normal heating within 10 to 15 minutes, the defrost cycle is doing its job.
When to worry. If the whoosh is followed by an hour of cool air coming through the registers, the defrost cycle is stuck. That’s a defrost board or sensor problem and needs a tech.
2. Loud bang or thud at startup or shutdown
A heat pump that bangs at startup, shutdown, or when the reversing valve switches usually has one of three problems:
- Failing compressor mount. The rubber feet that isolate the compressor from the cabinet have hardened or torn, and the compressor is now striking the cabinet. Common on units past 10 years, especially in coastal areas with salt-air exposure.
- Liquid slugging. Liquid refrigerant or oil is entering the compressor on startup, which makes a hydraulic bang. Caused by low refrigerant, a stuck expansion valve, or a flooded compressor after a long off cycle.
- Failing reversing valve. A bang during the mode switch (heat to cool or cool to heat) can mean the valve solenoid is sticking. The valve slams instead of sliding.
What to do. Don’t let the system keep banging. A loose compressor that strikes the cabinet repeatedly can crack a refrigerant line and dump the charge. Shut it off at the thermostat and call.
Typical 2026 SD costs. Compressor mount kit $200-$400. Reversing valve solenoid $250-$450. Slugging diagnosis runs $89-$150 for the service call plus whatever the underlying cause needs.
3. Grinding or rattling from the outdoor unit
The outdoor fan sits an inch or two from the coil and spins fast. Anything that gets between the blade and the cabinet, or anything that loosens on the blade, produces a hard mechanical noise.
The most common causes we see in San Diego:
- Debris in the fan blade. Palm fronds, eucalyptus bark, jacaranda blossoms, and bougainvillea leaves are all common. Inland yards see more debris from Santa Ana wind events.
- Loose fan blade. The setscrew that holds the blade to the motor shaft has backed off. The blade wobbles and strikes the shroud.
- Failing fan motor bearings. Bearings produce a steady grinding that gets worse as the motor heats up.
What to do. Shut the system off at the breaker before touching anything. Pull visible debris out by hand with the power off. If the noise comes back when you restart, the blade or motor is the problem. That’s a tech call.
Typical 2026 SD costs. Debris cleanout during a service call $89-$150. Fan motor replacement $350-$650 depending on the brand. Fan blade replacement $150-$300.
4. Squealing or high-pitched whine at startup
A high-pitched squeal that lasts only at startup and then quiets usually means motor bearings are starting to fail. The squeal is the bearing surface losing its lubrication film.
If the squeal is constant rather than just at startup, the bearing is further along. You usually have weeks rather than months before the motor seizes.
What to do. Schedule a tech within the week. A bearing replacement is sometimes possible but usually the motor gets replaced as a unit. Letting it run to failure can take out the motor capacitor and sometimes the contactor too, which adds $80-$200 to the repair.
Typical 2026 SD costs. Outdoor fan motor $350-$650. Indoor blower motor $400-$800.
5. Electrical hum with no airflow
If you hear a buzzing or humming from the outdoor unit but the fan isn’t spinning, the compressor is trying to start but can’t. Most common cause is a failed run capacitor. Second most common is a seized motor.
What to do. Shut the system off at the thermostat immediately. A motor that hums without spinning is overheating, and a long hum can burn out the windings. That turns a $200 capacitor job into a $1,000+ motor or compressor job.
Typical 2026 SD costs. Run capacitor $180-$280. Contactor $200-$350. Motor failures vary widely.
6. Hissing near the refrigerant lines
A hiss or faint whistle near the copper lines running between the indoor and outdoor units usually means refrigerant is escaping. In coastal San Diego homes, this is the failure pattern we see most often on units past 8 years because salt air corrodes the coil’s thin aluminum fins from the outside.
What to do. Call for a leak detection visit. Don’t add refrigerant without finding the leak first because the new charge will leak out within weeks or months and you’ll have paid for nothing.
Typical 2026 SD costs. Leak detection $250-$450. Leak repair plus recharge $400-$1,500 depending on where the leak is. If the leak is in the indoor or outdoor coil itself, a coil replacement runs $1,400-$2,800 and is often the moment to weigh a full system replacement.
Decision framework: call now vs. monitor
Call same day.
- Loud banging that’s new
- Grinding from the outdoor fan
- Constant electrical hum with no airflow
- Hissing near refrigerant lines
Call within a week.
- High-pitched squeal at startup
- New rattle that doesn’t go away
- Any noise paired with weaker airflow or weaker heating/cooling output
Monitor only.
- Whoosh during defrost mode change
- A faint click when the system starts or stops (relay contactor, normal)
- Soft hum from the outdoor unit while running (normal)
San Diego-specific noise patterns
Heat pumps in San Diego run more hours per year than air conditioners in most other US climates because we use them for cooling May through October and also for mild heating December through March. That’s why bearing failures and reversing valve issues show up here at lower ages than the national average. National averages assume a unit sits idle six months a year, which doesn’t match how heat pumps actually work in this climate.
We also see more salt-air corrosion noise complaints, including refrigerant line hissing and grinding fan bearings, in homes within about three miles of the coast. The salt accelerates aluminum and copper corrosion on outdoor components.
If your heat pump is approaching 10 years and you’re hearing new noises every few months, the repair-vs-replace math often tilts toward replacement, especially with current rebates. See our 2026 heat pump rebate stack guide for the cost side. If you’re seeing other failure signs alongside the noise, our heat pump not heating diagnostic walks through the parallel troubleshooting path.
FAQs
Is it normal for my heat pump to make noise during defrost?
Yes. A whoosh, brief steam plume from the outdoor unit, and a 5 to 15 minute pause in warm air are all normal during defrost. The cycle usually runs every 30 to 90 minutes in cold weather and lasts under 15 minutes.
Why does my heat pump bang when it turns on?
The three common causes are a failing compressor mount, liquid slugging in the compressor, or a sticking reversing valve. None of them are safe to ignore. A repeated bang can crack a refrigerant line and dump the charge, which turns a small repair into a system replacement.
What does a bad reversing valve sound like?
A bad reversing valve usually makes one of three sounds: a loud hammer or bang during the mode switch, a continuous hiss while the system runs, or no sound at all paired with the system stuck in one mode (only heats or only cools).
Should I shut off my heat pump if it’s making a grinding noise?
Yes. Shut it off at the breaker before checking the outdoor unit. A grinding fan blade can strike the coil and cause $2,000+ in damage in minutes. With the power off, pull any visible debris by hand. If the grinding returns at restart, call a tech.
How long can I run a heat pump that’s squealing?
A startup squeal usually buys you weeks before the bearing fails completely. A constant squeal during operation gives you days. Letting it run to failure often damages the capacitor and contactor too, so scheduling within the week is the right move.
Why is my heat pump noisier in winter than summer?
The reversing valve activates more often in winter (heating mode and defrost cycles), which produces audible mode-change sounds that don’t happen in summer cooling. The outdoor fan also runs more total hours in winter than in summer in San Diego because heating cycles are longer and more frequent than cooling cycles on mild days.
When to call us
If the noise is mechanical, loud, and new, shut the system off and call. Same-day diagnostic visits are $89 flat in San Diego County, credited toward the repair if you move forward. Call (442) 777-6440.